Reducing the environmental impact of coatings compositions, in particular that associated with emissions into the air of volatile organic compounds during their use, has been an area of ongoing investigation and development in recent years. Accordingly, interest in powder coatings has been increasing due, in part, to their inherently low volatile organic content (VOC), which significantly reduces air emissions during the application process. While both thermoplastic and thermosetting powder coatings compositions are commercially available, thermosetting powder coating compositions are typically more desirable because of the superior physical properties, such as hardness and solvent resistance, provided thereby.
Low VOC coatings are particularly desirable in a number of applications, such as the automotive original equipment manufacture (OEM) market, industrial market, and appliance market, due to the relatively large volume of coatings that are used in such markets. In addition to the requirement of low VOC levels, many manufacturers have strict performance requirements of the coatings that are used. In the case of basecoats, examples of such requirements include good corrosion resistance, substrate adhesion, and overcoat adhesion. In the case of topcoats, examples of such requirements include good corrosion resistance, adhesion (to undercoats and/or clear coatings applied thereover), exterior durability, solvent resistance, gloss, and appearance. While liquid coatings can provide such properties, they have the undesirable drawback of higher VOC levels relative to powder coatings, which have essentially zero VOC levels.
Curable powder coating compositions are available in a number of chemistries, such as: powder coating compositions that include epoxide functional polymer and epoxide reactive crosslinking agent; carboxylic acid functional polymer and betahydroxyalkylamide functional crosslinking agent; and hydroxyl functional polymer and capped isocyanate functional crosslinking agent. Presently available curable powder coating compositions can be subject to undesirable properties, such as insufficient storage stability at room temperature.
It would be desirable to develop new curable powder coating compositions that provide coatings having performance properties that are at least the same as those of presently available liquid and powder coating compositions. It would be further desirable that such newly developed powder coating compositions also possess at least a sufficient degree of storage stability at room temperature.